1. Field of the Invention
The present invention relates to a novel switching element which serves to Switch an electrical circuit from an off state to an on state or from an on state to an off state and, in one aspect, achieves such switching when rays of light (such as visible rays, ultraviolet rays, infrared rays and so on) are applied thereto.
2. Description of the Prior Art
Hitherto, photoconductive cells in which an ohmic contact electrode is provided on a photoconductor such as CdS, ZnO or the like, as well as optical switch diodes having p-n-p-n junctions, have been conventionally used in a wide range of fields as photoelectric conversion devices which serve to switch electrical circuits from an off state to an on state when light or an optical pulse is applied thereto. These photoconductive cells, however, have a disadvantage in that the switching speed they operate at is as low as about 100 msec. On the other hand, p-n-p-n optical switch diodes which perform on-off actions by utilizing the photoelectromotive force that is produced when light is applied to a p-n junction of a semiconductor exhibit a switching speed of 0.05 to 0.10 .mu.sec.
In these conventional elements, however, current continuously flows through them owing to their self-holding function once they have been turned on even if the irradiation of light is stopped. It is necessary, if a return to the off state is desired, to reduce the circuit current by changing certain external conditions.
Other optical switching elements having a semiconductor hetero-structure superlattice have been recently formed on an experimental basis, in which different types of semiconductors are laminated in alternate layers to provide a long-period structure (D. A. B. Millers, IEEE Journal of Quantum Electronics, QE-21, 1462 (1985)). Such optical switching elements are capable of rapidly switching electrical circuits in response to the application and removal of radiation. Materials used for forming the above-described type of semiconductor hetero-structure superlattice are, however, limited to such inorganic materials as GaAs, Si and the like, and the formation of such a superlattice requires a complicated process. No optical switching elements made of organic materials have been so far reported.